Evidence for an earliest Oligocene abrupt cooling in the surface waters of the Southern Ocean

Quantitative analysis of upper Eocene-upper Oligocene calcareous nannofossil assemblages from five Ocean Drilling Program sites in the Atlantic and Indian Ocean sectors of the Southern Ocean reveals an abrupt increase in cool-water taxa at the top of magnetic Subchron C13R ca. 35.9 Ma, coincident with an enrichment of ∼1‰δ¹⁸O in the planktonic foraminifers at these sites. The synchrony of the abrupt increase in cool-water taxa in the Southern Ocean renders this event a useful biostratigraphic datum at southern high latitudes. This earliest Oligocene cool-water taxa increase was the sharpest and largest during the late Eocene-late Oligocene interval and indicates a drop in surface-water temperature of more than 3 °C in the Southern Ocean. This suggests that the earliest Oligocene δ¹⁸O shift represents primarily a temperature signal; a small portion (∼0.2‰) is attributable to a global ice-volume increase.